The objective of this work is the understanding of acclimatization of man to life at high altitude. During the first expedition to Peru, oxygen dissociation curves, spirometry, arterial blood gases, and historical and physical examination data were gathered from a group of 50 natives from the mining town of Morococha (4,530 M). Considerable conflict existed as to the role of whole blood oxygen affinity in adaptation to life at this altitude. We found that natives of this community were considerably more alkalotic than was previously thought. Hyperventilation and alkalosis leads to high 2,3-DPG concentrations in red cells and whole blood p50 values which are 5-6 mm higher than sea level controls and suggests that oxygen affinity of blood in these patients is determined by many factors not just chronic hypoxia. Some individuals develop hemoglobin concentrations in the range of 24-25 gm% and suffer symptoms similar to those found in polycythemia vera patients at sea level.